It is known that high impact strength plastics can be prepared by polymerizing vinylidene group-containing monomers in the presence of elastomers. This polymerization can be carried out in a latex system; in a bulk system; by a combination method in which the vinylidene group-containing monomers containing the rubbery polymer dissolved therein are polymerized to a low degree of conversion and the partially polymerized mixture emulsified to form a latex, or put into a suspension, for the remainder of the polymerization. In these prior art processes the elastomeric component generally has been crosslinked before or during the polymerization of the vinylidene group-containing monomers.